finesse 

of a spectrometer
https://doi.org/10.1351/goldbook.08602
Free spectral range divided by resolution.
Source:
PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 747 (https://doi.org/10.1515/pac-2019-0203)